Update docs/config/screen.md

docs/config/screen.md

+# Touchscreen
+
+## Display resolution
+
+By default, a too high resolution is set and the embedded display does not support it.
+
+* See available modes ```xrandr```
+* Set mode: ``` xrandr -s 800x600 -r 75```
+* Make mode persistent: ```echo "xrandr -s 800x600 -r 75" >> .xprofile```
+
+## Touchscreen calibration
+
+* Install calibration tool, ```sudo apt install xinput-calibrator```
+* Run the tool, ```xinput_calibrator```
+* Follow screen instructions, clicking the screen on the target points.
+* Copy the calibration result in ```/usr/share/X11/xorg.conf.d/99-calibration.conf```, for example:
+
+    ```
+    Section "InputClass"
+    Identifier      "calibration"
+    MatchProduct    "Hampshire Company TSHARC Octopus"
+    Option  "Calibration"   "3981 186 152 3906"
+    Option  "SwapAxes"      "0"
+    EndSection
+      ```
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